KCSE 2012 Chemistry Paper 3 with Marking Scheme

1. You are provided with:
   -solution A containing an oxidizing agent A;
   -solution B 0.05 M aqueous sodium thiosulphate:
   -solution C containing a reducing agent C;
   -aqueous potassium iodide:
   -solution D, starch solution.
   You are required to determine the ;
   -Concentration of solution A:
   -Rate of reaction between the oxidizing agent A and the reducing agent C.
   
   Procedure I
   1. Using a pipette and a pipette filter, place 25.0cm³ of solution A into 250 ml conical flask.
   2. Measure 10cm³ of aqueous potassium iodide and add it to solution A in the conical flask. Shake the mixture. Add 10cm³ of 2 M sulphuric (VI) acid to the mixture and shake
   3. Fill a burette with solution B and use it to titrate the mixture in the conical flask until it just turns orange-yellow. Add 2cm³ of solution D to the mixture in the conical flask. Shake thoroughly. Continue titrating until the mixture just turns colorless. Record your results in the table 1 below.
   4. Repeat the procedure and complete table 1. Retain the remainder of solution A and solution D for use in procedure II.
      
      
      1. Calculate the:
         1. Average volume of solution B used : ( 1 mark)
         2. Number of moles of sodium thiosulphate: ( 1 mark)
            
            
      2. Given the one mole of A reacts with six moles of sodium thiosulphate, calculate the:
         1. Number of moles of A that were used; ( 1 mark)
         2. Concentration of solution A in moles per litre. (2 marks)
            
            

   Procedure II

   1. Label six test - tubes as 1, 2, 3, 4, 5, and 6 and place them in a test - tube rack.
   2. Using a clean burette, measure the volumes of distilled water shown in table 2 into the labeled test – tubes.
   3. Using a burette, measure the volumes of solution A shown in table 2 into each of the test-tubes.
   4. Clean the burette and rinse it with about 5cm³ of solution C.
   5. Using the burette, measure 5 cm³ of solution C and place it into a 100 ml beaker.
   6. Using a 10 ml measuring cylinder, measure 5 cm³ of solution D and add it to the beaker containing solution C. Shake the mixture.
   7. Pour the contents of test – tube number 1 in the beaker and immediately start a stop watch. Swirl the contents of the beaker. Record the time taken for a blue color to appear in table 2.
   8. Repeat steps 5 and 7 using the contents of test – tube numbers 2,3,4,5 and 6.
   9. Complete table 2 by computing Rate = ¹/_time(S^-1)
      
      (6 marks)
      
      
      
      1. Plot a graph of rate ( y-axis) against volume of solution A. (3 marks)
         
         
         
         
      2. What time would be taken for the blue colour to appear if the experiment was repeated using 4 cm³ of distilled water and 6 cm³ of solution A? (2 marks)
         
         
2. You are provided with solid E. Carry out the experiments below. Write your observations and inferences in the spaces provided.
   1. Place all of solid E in a boiling tube. Add about 20 cm³ of distilled water and shake until all solid dissolves, label the solution E. Use solution E for experiments (i) and (ii).
      1. To 2 cm³ of solution E, in a test – tube in each of the experiments I,II,III and IV, add:
         
         
         1. two drops of aqueous sodium sulphate;
            Observation inferences
            (1 mark) (1 mark)
            
            
         2. five drops of aqueous sodium chloride:
            Observations inferences
            (1 mark) (1 mark)
            
            
         3. Two drops of barium nitrate:
            Observations inferences
            (1 mark) (1 mark)
            
            
         4. two drops of lead (II) nitrate;
            Observations inferences
            ( 1 mark) (1 mark)
            
            
      2. To 2cm³ of solution E, in a test - tube, add 5 drops of aqueous sodium hydroxide. Add the piece of aluminium foil provided to the mixture and shake. Warm the mixture and test any gas produced with both blue and red litmus papers.
         Observations inferences
         ( 1 mark) (1 mark)
         
         
         
3. You are provided with solid F. Carry out the following tests. Write your observations and inferences in the spaces provided.
   1. Place all of solid F in a boiling tube. Add about 20 cm³ of distilled water and shake until all the solid is dissolved. Label the solution as solution F.
      Add about half of the solid sodium hydrogen carbonate provided to 2 cof solution F.
      Observations inferences
      (1 mark) (I mark)
      
      
   2. 1. Add about 10 cm³ of dilute hydrochloric acid to the rest of solution F in the boiling tube. Filter the mixture. Wash the residue with about 2 cm³ of distilled water. Dry the residue between filter papers. Place about one third of the dry residue on a metallic spatula and burn it in a Bunsen burner flame.
         Observation inferences
         (1 mark) (1 mark)
         
         
      2. Place all the remaining residue into a boiling tube. Add about 10 cm³ of distilled water and shake thoroughly. Retain the mixture for the tests in (C).
         Observations inferences
         (1 mark) (1 mark)
         
         
   3. Divide the mixture into two portions;
      1. To the first portion, add the rest of solid sodium hydrogen carbonate.
         Observations inferences
         ( 1 mark) ( 1 mark)
         
         
      2. To the second portion, add two drops of bromine water.
         Observations inferences
         (1mark) (1 mark)

MARKING SCHEME

1. Table 1
   
   

   	I	II	III
   Finial burette reading	17.45	32.90	36.05
   Initial buretter reading	2.10	17.45	20.60
   Volume of solution B used (cm3)	15.35	15.45	15.45

    (4 marks)
   1.  
      1. Average volume
         = 15.35 + 15.45 + 15.45
                           3
         =15.42cm³     (1mark)
      2. Moles of sodium thiosulphate used
         = 0.05 × 15.42
                10000
         7.71 × 10⁻⁴ moles  (1 mark)
   2.  
      1. Number of moles in A 25.0cm³
         mole ratio A:Na₂S₂O₃.5H₂O
                         1:6
         7.71 × 10⁻⁴ = 1.28 × 10⁻⁴ moles    (1 mark)
               6
      2. Concentration of solution A in mol dm³
         1.28 × 10⁻⁴moles in 25cm³
             ?             moles in 1000cm³
         1.28 × 10⁻⁴ × 1000
                      25
         5.12 × 10⁻³ moles/dm⁻³     (2 marks)
         
         Table 2 
         
         

         Test tube number	1	2	3	4	5	6
         Volume of distilled water (cm3)	0	2	3	5	6	7
         Volume of solution a (cm3)	10	8	7	5	4	3
         Time (s)	22.5	28.0	32.0	50.0	57.5	85.0
         Rate = 1/Time (s−1)	0.044	0.036	0.031	0.020	0.017	0.012

         (6 marks)
         1. Graph of rate  (3 marks)
            
            
            
         2. Time taken for 4cm³ of distilled water.
            ∴ 6cm³ of solution A is added 
            from the graph = 25 × 10⁻³ sec⁻¹
                                  = 40 seconds
2. Observation 
   

   (a) (i)	(I)	A white precipitate (1)	Presence of Pb2+, Ba2+ or Ca2+  1 mark for all the 3 ions  ½ mark for 2 correct ions  0 mark for one or none
   	(II)	No white precipitate (1)	Absence of Pb2+   (1)
   	(III)	No whit precipitate (1)	SO42−, SO32−, CO32− ions absent   (1)  1 mark all the 3  ½ mark for ions correct  0 mark for one or none
   	(IV)	No white precipitate (1)	Cl− ions absent  (1)
   (iii)		Effervescence ½/Bubbles/Frizzing Colourless gas produced ½ Turns red litmus blue ½ Blue litmus remained blue ½   (2 marks)	NO3−  present (1)
   			( Total 11 marks)

3.  
   

   	Observations	Inferences
   (a)	No effervescence (1)	Compound/solution F not acidic H+ or R-COOH absent.   (1)
   (b) (i)	Burns with a sooty/smoky ½ Luminous /yellow flame ½	Unsaturated cpd (1)   Long chain hydrocarbon or
   (ii)	Some white suspension/solid remains undissolved  ½	Compound slightlt/ partially soluble in water  ½
   (c) (i)	Effervescence ½  Colourless gas produced ½	Mixture is acidic   (1)  RCOOH present
   (ii)	Not decolourized   (1)	absent   (1)       absent

                                                                                                               (Total = 9 marks)